Environmental Geology 1/3

What is porosity?

The amount of pore space within a rock or sediment, expressed as a percentage

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What is permeability?

The ability of a rock to transmit fluids such as water, oil and gas, expressed as a rate of flow

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4 factors which affect the porosity of sedimentary rocks

1. Degree of sorting 2. The amount of diagenesis the rock has undergone 3. Grain shape 4. The packing of the grains and how they fit together

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What is the porosity in igneous?

They have virtually 0 porosity due to the amount of diagenesis that they have udnergone

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How do rocks develop secondary porosity?

Through structures in the rock such as joints and faults. Only effective if the pore spaces are interconnected

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2 factors affecting permeability

1. If the rock has high effective porosity with good interconnections it will have a high permeability 2. Coarse grained rocks have a higher permeability as there is less resistance to flow around coarse grains

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What is secondary permeability?

Where the rock has fractures, joints, voids, caves produced by the solution

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Name 3 rocks that are impermeable

1. Clay 2. Mudstone 3. Shale

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What is groundwater?

Water retained in the pore spaces of rocks below the water table, usually originates from rainwater that has infiltrated into soil and then percolated downwards through pore spaces to reach the water table

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What is the water table?

The surface separating unsaturated rock above from saturated rock below

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Where are unsaturated rocks located?

Above the water table, they have air and water in their pore spaces

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Where are the saturated rocks located?

Below the water table and only contain water in their pore spaces

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What factors affect the shape of the water table?

1. The surface topography 2. The season 3. How much rainfall the area recieves

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What is hydrostatic pressure?

The pressure at a point in a body of water due to the weight of the overlying column of water

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What is the hydrostatic head?

The height of the overlying column of water

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What is the hydraulic gradient?

The difference in hydrostatic pressure between two points divided by the distance between them

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How does the water flow?

Always flows down the hydraulic gradient- areas of high pressure to low pressure. The rate at which he water flows is proportional to the drop in height of the water table

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Where is groundwater stored?

Aquifers

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What is an aquifer?

A body of porous and permeable rock capable of storing and yielding significant amounts of water

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Do aquifers have high or low porosity? Why?

High- so that water can be stored in their pore spaces

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Do they have high or low permeability? Why?

High- so that water can enter, flow through and be extracted from the aquifer with ease

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Name 3 suitable rocks for an aquifer

1. Poorly cemented sandstones 2. Limestone 3. Fractured chalk

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Name a case study of aquifer rocks

British isles are chalk

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What is a recharge zone?

Area of an aquifer open to the atmosphere allowing replenishment of water, usually rainwater and can provide a constant supply of water provided, the rate or recharge equals the rate of extraction

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Name the two types of aquifer

1. Confined aquifer 2. Unconfined aquifer

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What is a unconfined aquifer?

Open to the atmosphere, under atmospheric pressure and is recharged by rainwater from directly above, water will need to be pumped to the surface from a well or borehole sunk into an unconfined aquifer

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What is an confined aquifer?

Overlain by impermeable rocks and the groundwater held within is under hydrostatic pressure, groundwater can only be replenished in a confined aquifer if it has recharge ones that are open to the atmosphere

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What is an aquiclude?

An impermeable rock that does not transmit water

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What is a perched aquifer?

Sits above the regional water table and is underlain by a lens of impermeble rock which prevents the water from percolating further downwards

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What is a live aquifer?

One that is currently being replenished by rainwater via a recharge zone on the surface

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What is a fossil aquifer?

No longer being replenished and represents a relic of past wetter climate. They are non-renewable

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Give one example of a fossil aquifer

Ogallala aquifer of central USA

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What is an artesian basin?

A large, synclinal confined aquifer under hydrostatic pressure

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What is an artesian well?

Holds water under hydrostatic pressure, which rises up the well on release

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What is abstraction?

The removal of water from any source

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How is groundwater abstracted?

Sinking a borehole and pumping the water to the surface, as water is pumped out of the ground from a well, the level of the water table falls around the well leaving a cone of depression

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What is the draw down?

The height difference between water table and the water level in well

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Where wil the greatest pressure be in the aquifer?

The part with the highest hydrostatic head

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3 problems caused by groundwater extraction

1. Lowering of the water table 2. Subsidence at the surface resulting form the removal of water from the pore spaces of the rocks 3. Saltwater encroachment in coastal areas

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How can the lowering of the water table be a problem?

Shallow wells become dry and have to be sunk deeper- if the wells are situated close together, their cone depressions can overlap which leeds to the lowering of the water table

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How can subsidence cause a problem?

Rocks overlying the aquifer collapse downwards creating depressions at the surface which can be several metres in diameter, subsidence results results in compaction of the aquifer and a permanently reduced water storage capacity

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How can saltwater cause a problem?

Less dense fresh groundwater forms a lens floating on top if the more dense sea water, over-pumping then disturbs the fresh water-saltwater interface and allows the sea water to enter the aquifer, the water becomes saline and unfit for drinking

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Name 2 threats to groundwater

1. Over-pumping 2. Pollution

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What is over-pumping?

If too much groundwater is extracted then there may not be enough left to provide a reliable public water supply

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How can pollution be a threat?

Groundwater is vulnerable to contamination from a variety of sources and once polluted is difficult to clean. Unconfined aquifers are more at risk

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What is the natural filtration process?

Where the rocks act as a natural filter removing impurities from the water- the natural filtraion process not only removes chemical impurities but viruses and bacteria, this means that the groundwater does not always require treatment

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How do soluble minerals get into the water?

As the water passes through the rock, they are dissolved and taken into solution, they can make the water taste good or bad and can be beneficial or harmful

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What does 'hard' groundwater contain?

Calcium and magnesium. It is not harmful to drink but leaves limescale in kettles and can be difficult to lather when you add soap

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How can fluoride affect drinking water

In drinking water it can prevent tooth decay but too much can lead to the teeth becoming discoloured or pitted and it can cause more serious health problems such as joint pain, liver pain and kidney disease

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What are the sources of groundwater pollution?

1. Nitrates, pesticides and microbes from agricultural run-off and sewage 2. Hydrocarbons and solvents from petrol stations and factories 3. Toxic fluid from landfill waste disposal sites 4. Acid mine drainage water containing toxic metals

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What are springs?

Springs occur when the water table intersects the land surface and groundwater flows out onto the surface rocks, this often occurs at the junction between permeable and impermeable rocks

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What is a spring line?

A line of springs occurring along a boundary of permeable and impermeable rocks

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Name the three types of springs

1. Lithological spring 2. Springs at faults 3. Springs at unconformities

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What is a lithological spring?

Result from changes in the rock type. They occur where porous and permeable rock overlies impermeable rock, the water table will intersect the land surface at the junction between the two rock types, there will be a spring line along permeable rock

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What are springs at faults?

Faults can produce springs if they have moved porous and permeable rock into contact with impermeable rock, a spring line will occur where the fault plane intersects the land surface. Faults can result in the formation of pressurised springs

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What is a springs at unconformities?

If porous and permeable rock lies uncoformably on top of impermeable older rock, the water table will intersect the land surface at the junction between the two rock types, a spring line will occur where the plane of the unconformity intersects land

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Name a case study of springs

Bath- famous for its thermal springs

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Name 4 advantages of surface water supply

1. Easy to abstract water from rivers, lakes, reservoirs by direct pumping 2. Water can be treated after use and put back into a river 3. Dams can be used to generate hydroelectric power 4. Reservoirs can be used for other purposes

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Name 4 disadvantages of water supply

1. Water needs treatment 2. Reservoirs will eventually silt up 3. Construction dams and reservoirs can trigger earthquakes 4. It can be expensive and environmentally damaging

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Name 4 advantages of groundwater supply

1. Rocks act as a natural filter purifying the water 2. There is no loss of water through evaporation 3. No requirement for expensive dams 4. If its from an artesian basin, pumping costs will be low

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Name 4 disadvantages of groundwater supply

1. Requires sedimentary rocks and presence of aquifers 2. Problem of surface subsidence 3. Pollutants have long residence time 4. Groundwater not always suitable for drinking

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What is a dam?

A structure that holds back water

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What is a reservoir?

Body of water behind a dam

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What is hydroelectric power?

Releasing water stored behind a dam to turn a turbine to generate electricity

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What is a renewable resource?

One that is replenished by natural processes at a rate equal to or exceeding its rate of use

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What does sustainable mean?

A resource is sustainable if it is used in such a way that can continue into the future

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What type of aquifer has a renewable resource?

Live aquifer

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How do you know whether a groundwater supply is sustainable?

Only sustainable if the at of extraction does not exceed the rate of replenishment. Groundwater supplies are only sustainable if the natural system of filtration through rock can clean the water from pollutants

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Name a non-renewable aquifer

Ogallala aquifer, USA

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What is artificial recharge?

Water is pumped into the ground through boreholes or by using controlled flooding to spread water over a large area so that it can infiltrate the ground to store surplus water in aquifers for future use

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Name an example where artificial recharge is used

London Basin are recharged with treated river water during the winter months

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What is petroleum?

A liquid mixture of hydrocarbons present in suitable rock strata

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What is the main use of petroleum?

It is used as a fuel and it forms the basis of the multi-million pound petrochemicals industry

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What resource are crude oil and natural gas?

Non-renewable

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Where do oil and natural gas originate?

Sedimentary basins

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What are the main requirements to form economic accumulations of oil and natural gas and oil?

1. Source rock 2. Maturation 3. Migration 4. A reservoir rock 5. A cap rock 6. A trap

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What is a source rock?

Organi-rich mudstone or shale containing abundant plankton that formed in low energy, anoxic, marine conditions. Their dark colour reflects their high organic carbon content

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What is maturation?

A process where plankton is converted into petroleum by the effects of temperature an pressure during burial

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What is migration?

Describes the movement of petroleum from a source rock to a reservoir rock

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What is a reservoir rock?

Highly porous and permeable rock capable of storing and yielding significant amounts of petroleum

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What is a cap rock?

Impermeable rock above the reservoir rock preventing further upwards migration of petroleum

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What is a trap?

Geological situation that concentrates petroleum in one place

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What are oil and natural gas formed form?

The remains of microscopic marine organisms called plankton

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How do source rocks form?

Organisms floating in the water column die and then sink down through the water column and accumulate on the seabed, low energy environment mans the plankton settle on the anoxic seabed so they do't decay, they the decay and form sapropel

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What sediment is needed for good burial?

Fine grained sediment results in the formation of organic-rich sedimentary rock

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What happens when the source rock is buried?

It is subjected to compaction and due to the geothermal gradient the temperature increases, over time the organic matter breaks down to form the organic compounds of carbon, hydrogen, nitrogen and sulfur called Kerogen and then petroleum- maturation

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What temperatures does petroleum form at?

50-200 degrees C

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What temperatures does oil form at?

50-100 degrees C

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What temperature does natural gas form at?

100-200 degrees C

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What happens above 200 degrees?

The hydrocarbons denature and get destroyed

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What happens when the petroleum is formed?

It undergoes migration from the source rock to the reservoir rock

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What are the 4 factors that affect migration?

1. Permeability of the rocks 2. Pressure 3. Density differences 4. Viscosity of the oil

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Name 3 suitable reservoir rocks

Poorly cemented sandstones, most limestones and fractured chalk

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What happens if there is no cap rock present?

The oil and natural gas will continue to rise, eventually forming oil seeps and tar pits on the surface

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Name a 4 suitable cap rocks

1. Clay 2. Mudstone 3. Shale 4. Evaporites- crystalline sedimentary rocks

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What do all traps require?

Presence of porous and permeable rock overlain by an impermeable cap rock

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Where does gas always form in a trap?

Gas always forms a horizontal layer above the oil

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Where doe oil form in a trap?

Oil form a horizontal layer above the water in the pore space of the reservoir rock

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Name the 5 types of traps

1. Anticline traps 2. Fault traps 3. Salt dome traps 4. Unconformity traps 5. Lithological traps

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What is an anticline trap?

Form when a rock is folded into arch shapes, oil and natural gas will be concentrated at the crest of the anticline provided by overlying cap rock, the larger the fold the more reserves, once filled gas/oil can leak out laterally-migrate rocks around

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What is a fault trap?

Form when movement along a fault plane result in a reservoir rock being moved adjacent to an impermeable rock-this stops the contents from leaking laterally, the fault must be sealed to stop contents from escaping, if strata are dipping-migration up

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What are salt dome trap?

Form from the presence of evaporites such as halite and gypsum, due to their low density than surroudning rock they rise upwards towards the surface, typically 1-10km in size, as the evaporites are crystallin they are impermeable-form good cap rock

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What are unconformity traps?

Form if the reservoir rocks below an angular unconformity are overlain by cap rocks above the unconformity, oil/gas will migrate up and be trapped beneath the unconfomity, the reservoir rock beneath must have impermeable rocks either side

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What are lithological traps?

Result in variation of rock type, fossilised limestone reefs make good lithological traps if surrounded by impermeable rock, as they form in areas where there is life they produce good organic matter for petroleum formation

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How does the temperature exceed 200 degrees which destroys the oil and gas?

1.Heat from igneous intrusion or volcanic activity 2. Regional metamorphism 3. Burial where the geothermal gradients goes above 200

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How can oil and natural gas be lost from a trap?

1. Erosian and removal of overlying cap rock 2. Escaping upwards along the unsealed fault plane

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Name two geophysical exploration techniques

1. Seismic reflection 2. Gravity surveys

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SEISMIC REFLECTION SURVEYS-How are the artificial seismic waves generated?

1. Explosions or vibrations on land and by air guns in water

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What are the reflected by?

Layer boundaries within the sedimentary sequences

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What happens to the waves next?

They travel back up to the surface where they are detected by Geophones on land or hydrophones at sea. Their location is accurately pinpointed by GPS

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What is an advantage of doing seismic surveys at sea?

Theya re efficient because a large number of hydrophones can be dragged behind a ship and the ships path is not restricted

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What is the travel time?

The time taken for the reflected waves to arrive back at the receivers

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What is the travel time used for?

To calculate the depth to the reflective layer

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What doe the data plot?

A seismic profile

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What do seismic profiles show?

The subsurface layering and can be interpreted by geophysicists to identify potential traps

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GRAVITY SURVEYS- What is a gravimeter?

Measures small variations in the Earths gravitational field strength. they can be mounted in road vehicles, helicopters or planes

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What is the unit of measurement?

Milligals (mGals)

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Why is the data then corrected?

For the affects of latitude, altitude and topography leaving the variation resulting from the underlying rock types

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How are the maps plotted?

Plotted with lines joining points of equal gravitational field strength and anomalies can be identified

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What is a gravity anomaly?

A departure from the normal value and may be positive or negative

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What does a positive anomaly result from?

An excess of mass, this may be due to the presence of an anticline or an uplifted block bounded by faults which could be potential trap structures

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What does a negative gravity anomaly result form?

A deficit of mass, this may be due to a low-density salt dome. In this case the exploration tagrte would be around the edge of the salt dome at the zero millegal line

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When is exploration drilling carried out?

When potential oil and natural gas traps have been located

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What is exploration drilling?

Holes are drilled with cylindrical bits studded with diamonds. The rotating drill bit is cooled and lubricated by drilling mud containing the mineral barite to make it dense

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What is a disadvantage of exploration drilling?

It can be expensive

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What is mud loggers?

Geologists who identify the rock type and microfossils present at different depths down the hole-this allows them to build up a picture of the changing rock types down the hole and to correlate the geology between boreholes

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What is down-hole logging?

A sonde with geophysical equipment is passed down the drill hole on a wireline and it records data as it is slowly pulled up

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Environmental Geology 1/3

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